The present invention relates to a recording medium, a recording apparatus and a recording method.
In the information society in recent years, the amount of information is on a sharp increase. Therefore, it is of very high importance to develop a recording method or a recording apparatus capable of drastically improving the recording density, compared with the prior art.
As a technology for realizing a high recording density, attentions are paid to an optical recording such as a heat mode recording in which light is converted into heat for performing the recording or a photon mode recording in which recording is performed without converting light into heat.
As a heat mode recording, a magneto-optic recording or a phase transition recording has already been put to a practical use. In such a technology, however, a drastically high recording density has not yet been realized. Therefore, in order to improve the recording density in such a heat mode recording, proposed is a technology utilizing a Near-field Scanning Optical Microscope (NSOM) capable of forming a recording mark smaller than a wavelength of light.
For example, it is reported in “Appl. Phys. Lett. 61, 142 (1992)” that Betzig et al. irradiated a Co/Pt multi-layered film with an output of an Ar ion laser by using an NSOM probe in an attempt to magneto-optically record/reproduce information, thereby forming a recording mark having a diameter of about 60 nm. It is also reported in “Thin Solid Films 273,122 (1996)” and “J. Appl. Phys. 79,8082 (1996)” that Hosaka et al. irradiated a Ge2Sb2Te5 thin film having a thickness of about 30 nm with an output of a semiconductor layer by using an NSOM probe, thereby forming a recording mark having a diameter of about 50 nm.
In these methods, however, the recording mark is enlarged by the diffusion of heat. In addition, the energy required for the recording is large. Therefore, it is considered impossible to realize a recording density on the order of terabits/cm2, which requires recording marks of about 10 nm.
Japanese Patent Disclosure (Kokai) No. 7-254153 discloses a recording medium for another heat mode recording, in which the recording is performed by utilizing the change in phase of an organic dye, and the recorded data is read out by detecting fluorescence. According to this recording medium, the recording mark can be diminished by using an organic dye molecule having a low heat conductivity. However, it is difficult to achieve a recording density on the order of terabits/cm2, which requires a recording mark of about 10 nm, in this method, too. Also, in this recording medium, the amorphous region corresponding to the recording mark is contiguous to the crystal region, with the result that the amorphous region tends to be crystallized. In other words, the recording tends to be broken. Further, since the recording layer has a uniform composition in the recording medium, the difference in signal intensity is small between the crystal region and the amorphous region, giving rise to a large noise.
The problem of noise generation takes place not only in the heat mode recording medium but also in the photon mode recording medium having a uniform composition in the recording layer.
Concerning the photon mode recording, Japanese Patent Disclosure No. 8-45122 discloses a recording medium, in which dot-like (domain structure) recording region consisting of an organic dye molecule and having a diameter of about 10 to 100 nm is formed on a substrate, and the recording is performed by injecting an electric charge into the recording region. In this recording medium, a single dot corresponds to a recording unit (1 bit), making it possible to achieve a high recording density. However, in this recording medium, it is difficult to form dot-like recording regions of a uniform size and to arrange regularly the dot-like recording regions. In addition, the size and arrangement of the dot-like recording regions tend to be changed with time. Further, since it is necessary to bring an electrode into contact with the dot-like recording region when an electric charge is injected into the dot-like recording region and when the electric charge is discharged from the dot-like recording region, it is necessary for that surface of the recording medium on which the dot-like recording regions are formed to be smooth. However, it is generally difficult to achieve the required smoothness. Also, even if the required surface smoothness is achieved by forming a protective layer on that surface of the recording medium on which the dot-like recording regions are formed, a new problem is brought about that a high voltage is required for injecting and discharging electric charges into and from the recording regions.
As described above, various problems remain unsolved in the conventional recording technology that it is impossible to form a small recording mark, that a large energy is required for the recording of information and for the reproduction of recorded information, that the recorded information tends to be broken, that a large noise is generated, and that it is difficult to make the surface of the recording layer smooth. Also, where a smooth surface is realized by forming a protective layer on the recording surface, a high voltage is required for the injection and discharge of the electric charges. Under the circumstances, a recording technology capable of achieving a super high recording density has not yet been put to a practical use.